The present invention relates to a baseball or softball bat that is formed of a composite material including wood and a fiber-reinforced plastic, and a method for manufacturing the same.
Conventionally, baseball or softball bats (hereinafter, referred to as bats) are manufactured by a single material such as wood, metal, or a fiber-reinforced plastic. In recent years, it has been desired to reduce the use of wood due to a lack of optimal wood resources or to improve the feeling of use at an impact. Therefore, instead of manufacturing bats with a single material, bats have been proposed that use wood at a batting portion and a fiber-reinforced plastic or metal at portions other than the batting portion and have a composite structure in which these different materials are joined. In this kind of bat, however, the strength at a joint surface between the different materials is insufficient. Thus, when stress caused by batting a ball with the batting portion acts on the joint surface, the joining state of the materials at the joint surface deteriorates.
To solve this problem, Japanese Laid-Open Patent Publication No. 2004-113557 proposes a bat that has improved shock resistance and durability. That is, a reinforcing member is provided on the outer circumference of the joint surface to add strength to the joint surface. According to the bat disclosed in Japanese Laid-Open Patent Publication No. 2004-113557, a bat main body 91, which is formed of a fiber-reinforced plastic into a hollow shape, is inserted in a batting portion 90 formed of wood into a cylindrical shape as shown in FIG. 7a. The bat main body 91 includes an insertion section and an exposed section, which are integrally formed. The insertion section includes a batting core portion 92, the outer circumference of which is covered by the batting portion 90. The exposed section, which is exposed from the batting portion 90, includes a tapered portion 93, a grip portion 94, and a grip end portion 95. As shown in FIG. 7b, a reinforcing member 96 is wound about a boundary portion between the batting portion 90 and the tapered portion 93. The reinforcing member 96 covers the section in the vicinity of the lower end of the batting portion 90 and the section in the vicinity of the upper end of the tapered portion 93.
The bat disclosed in Japanese Laid-Open Patent Publication No. 2004-113557 is molded as described below. The bat is placed in a metal mold in a state in which the batting core portion 92 of the bat main body 91 is inserted in the batting portion 90, and the reinforcing member 96 is wound about the outer circumference of the boundary portion between the batting portion 90 and the tapered portion 93. Then, compressed air is injected in the bat main body 91 of the bat placed in the metal mold before being molded so that pressure is applied to the bat main body 91 from the interior, and the metal mold is heated at the same time. In this manner, the bat main body 91, which is formed of a fiber-reinforced plastic, is joined to the batting portion 90, which is made of wood.
The lower end of the batting portion 90 has a thickness in the radial direction. Thus, a step 97 is formed at the boundary portion between the batting portion 90 and the tapered portion 93. Therefore, according to the bat molded as described above, the bat main body 91 is undesirably enlarged in the radial direction along the shape of the step 97 in the vicinity of the lower end of the batting portion 90 due to the pressure applied from the interior of the bat main body 91. As a result, the orientation of the fibers in the fiber-reinforced plastic is rapidly changed at sections above and below the boundary portion between the batting portion 90 and the tapered portion 93. That is, the orientation of the fibers in the fiber-reinforced plastic arranged in the longitudinal direction of the bat is significantly changed in the radial direction of the bat.
In such conventional bats, stress caused when batting a ball with the batting portion 90 is intensively applied to the vicinity of the boundary portion between the batting portion 90 and the tapered portion 93. Therefore, in such bats, the strength of the bat cannot be maintained in a suitable manner although the outer circumference of the boundary portion is covered by the reinforcing member 96. The reinforcing member 96 cannot efficiently improve the joining state of the joint surface of the different materials at the boundary portion, and such bats have problems with shock resistance and durability.